1. Field of the Invention
The present invention relates to an optical scanning device and a method of adjusting the optical scanning device, and more particularly, to an optical scanning device that scans a target surface with a light beam, a method of adjusting a lateral magnification and a focal position of an optical system included in the optical scanning device in a sub-scanning direction, and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
In recent years, in the field of image forming apparatuses, such as laser printers and digital copying machines, improvements in printing speed (speeding up) and scanning density (high-quality images) have been required. In response to these requirements, methods of scanning a target surface with a plurality of light beams using a multi-beam light source that can emit multiple light beams have been developed. As a result, various types of scanning optical systems that can operate with multiple light beams have been accordingly proposed.
Japanese Patent No. 3445050 discloses a multi-beam scanning optical system. In the multi-beam scanning optical system, a variation in F-number determined by an image height produced by a light beam incident on a target surface in the sub-scanning direction is reduced by sequentially varying, from on-axis toward off-axis, curvatures of both surfaces of a single lens in the sub-scanning direction.
Japanese Patent No. 3768734 discloses an optical scanning device that includes at least two scanning imaging lenses. The scanning imaging lenses have at least two special surfaces in which the curvature in the sub-scanning direction varies from the optical axis toward peripheral regions in the main-scanning direction. At least one surface of the special surfaces has a property in which a variation of curvature in the sub-scanning direction is asymmetrical with respect to the main-scanning direction and the curvature in the sub-scanning direction has a plurality of extreme values.
Japanese Patent Application Laid-open No. 2005-338865 discloses a scanning optical device that includes two lenses. By sequentially varying, from on-axis toward off-axis, curvatures of the surfaces of the two lenses in the sub-scanning direction, the position of the principal plane in the sub-scanning direction is made closer to a target surface in the optical-axis direction compared with a position thereof in the off-axis direction. Accordingly, in the scanning optical device, variation in the value of the F-number determined by the image height produced by a light beam incident on a target surface in the sub-scanning direction is reduced.
Japanese Patent Application Laid-open No. 2002-287055 discloses an optical scanning device that includes a light source and a deflector. The light source includes a plurality of light-emitting elements that are two-dimensionally arrayed. The light-emitting elements are used for scanning an identical or different scanning line. The deflector deflects multiple light beams emitted from the light source in the main-scanning direction at the same time. In the optical scanning device, the light source is made to rotate about the optical axis in such a manner that exposure energy distribution in the sub-scanning direction produced by light beams in an identical scanning region is substantially equal to a desired exposure energy distribution.
Japanese Patent Application Laid-open No. 2000-9994 discloses a collimator lens that includes a first lens and a second lens that are arranged in this order on the collimated light-beam side. The first lens has a concave surface on the collimated light-beam side and the second lens has a positive refractive power. At least one of the surfaces of the first lens is an aspheric surface.
When performing high density scanning, it is advantageous to use a light source, such as a surface emitting laser array, in which a plurality of light-emitting elements is two-dimensionally arrayed in high density.
Using a light source that includes a one-dimensional array in which a plurality of light emitting elements are arranged one dimensionally, it is possible to simultaneously adjust intervals of the light-emitting element in the sub-scanning direction by rotating the light source about an axis parallel to the light emitting direction (gamma rotation), whereby scanning-line intervals on the target surface can be made uniform. In this specification, the term “interval of the light-emitting element” refers to the distance between the centers of two neighboring light-emitting elements.
The problem is that when performing gamma rotation using the light source in which the light-emitting elements are two-dimensionally arrayed, the intervals of the light-emitting element varies (see FIG. 24). This makes it difficult to perform precise adjustment of the scanning-line intervals on the target surface in a uniform manner. This problem should be addressed in order to improve image quality that may be further required in the future.